The X800 *can* do frame buffer blending.. least, that's what I thought. As I understand it, it's just that the accumulation buffer (for blending) on the ATi cards doesn't have high enough precision for true OpenEXR HDR. At least, that's the way I understand it... but on that, I could be wrong, and would be happy to be corrected. I'm really not as knowledgeable about the R420 as I should be.
Edit: Apologies, it seems you're right. Looked it up, and the 420 can do true OpenEXR HDR... but it has to use pixel shaders to do so. I'm still fairly certain it can do a reduced qual version natively though.